Liquid phase process for the preparation of nitro derivatives



LIQUID PHASE PROCESS FOR THE PREPARATION F NITRO DERIVATIVES Charles Philip Spaeth, Woodbury, NJ., assignor to E. I. du Pont de Ne'mours and Company, Wilmington, Del.,

a corporation of Delaware No Drawing. Application April 16, 1951 a Serial No. 653,074

9 Claims. (Cl. 260-644) The present invention relates to a process for the preparation of nitro derivatives of alkanes, cycloalkanes, and aryl-substituted alkanes. More particularly, the present invention relates to a process for the liquid-phase nitration of alkanes, cycloalkanes, and aryl-substitutedalkanes by an alkyl nitrate.

.Nitro. derivatives of such hydrocarbons are useful as solvents and as chemical intermediates, particularly as intermediates for valuable amines. Heretofore, the nitro derivatives were prepared generally by processes involving the use of nitric acid as the nitrating agent. This nitrating agent, however, has not been entirely satisfactory. Not only does the corrosive action of the agent necessitate the use of expensive storage and handling facilities, but also in most nitrations by nitric acid, the presence of water gives rise. to the formation of large amounts of nitrites which are acted upon by the nitric acid to give large amounts of undesirable oxidation products, thus decreasing the yield of desired nitro compounds. In addition to the economic disadvantages incurred by the use of nitric acid, in many cases a catalyst must be used with these agents to bring about the desired results, and in the two-phase nitric acid-hydrocarbon system, vigorous agitation is required to provide the necessary contact of reactants. Moreover, in the nitric acid process'es of the prior art the substitution of the nitro group cannot be limited solely to the alkyl side-chain of an aryl-substituted alkane; in theseprocesses, the nitro substituent also is introduced on the aromatic ring, giving undesirable by-products and lower yields of the desired products.

Accordingly, an object of the presentinvention is .to provide a simple and economical process for the nitration of alkanes, cyclokanes, and aryl-substituted alkanes in which process nitrite formation is negligible. Another object of the present invention is to' provide a process whereby the hydrocarbons are nitrated in the absence of a catalyst. A further object of the present invention is to provide a one-phase nitration process in which vigorous agitation of the reactants is not required. A still further objectof the present invention is to provide a nitration process whereby nitro substituents can be introduced readily into the alkyl side-chain of aryl-substituted alkanes without attendant ring nitration.

I have found that the foregoing objects may be achieved when I react an alkyl nitrate with an alkane, cycloalkane, or aryl-substituted alkane at elevated temperature and pressure.

In accordance with the process of the present invention, an alkyl nitrate is admixed with a molar excess of an alkane, cycloalkane, or aryl-substituted alkane, and the mixture is maintained at a temperature within the range of 140 and 300 C. and a pressure of at least 125 p.s.i.ga. which is sufiicient to maintain said hydrocarbon substantially in the liquid phase.

The following examples serve to illustrate specific embodiments of the method of carrying out the process 2,883,434 Patented Apr. 21, 1959 of the present invention. However, they will be understood to be illustrative only and not as limiting the invention in any manner.

Example 1 Methylcyclohexane was introduced into a reactor which had previously been flushed with nitrogen. The reactor was pressurized to 400 p.s.i.ga. with nitrogen, and the methylcyclohexane was heated to 203 C. Then,-a mixture of n-propyl nitrate and methylcyclohexane was pumped into the reactor over a period of about 7 minutes,

' the temperature and pressure being maintained substantially at the initial levels. When the addition of the mixture was complete, additional methylcyclohexane was fed through the pump lines over a period of about two minutes to clear the lines of the nitrate-methylcyclohexane mixture. The mole ratio of total methylcyclohexane to n-propyl-nitrate used was 20. The product was cooled to about 30 C. and the reactor vented to atmospheric pressure. The product mixture was removed, washed with dilute sodium bicarbonate solution, and analyzed. The analysis indicated that a mixture of l-methyLlnitrocyclohexane and a secondary mononitro derivative in 70% total yield and oxidation products in 26% yield were obtained.

Example 2 Cyclohexane wastreated with n-propyl nitrate accord ing to the procedure of Example 1, and analysis showed that nitrocyclohexane in 53% yield was obtained. No nitrite was found in the product mixture.

Example 3 Example 4 Cyclohexane was treated with isopropyl nitrate in the manner described in Example 1, and nitrocyclohexane in 35% yield, as determined by analysis, was obtained.

Example 5 The procedure of Example 1 again was repeated with the exception that n-nonane and n-propyl nitrate in a mole ratio of 15 were used as the reactants. Analysis showed that a mixture of primary and secondary mononitro-n-nonanes in 34% total yield was obtained.

As may be seen by reference to the foregoing examples, the nitration process of the present invention proceeds smoothly and rapidly. to give nitro compounds as the major product without ring nitration and without the formation of appreciable quantities of undesired oxidation products. The nitration can be eflected at temperatures of at least 140 C., and good results are obtained at temperatures of up to about 300 C. At higher temperatures, tar is formed and yields are decreased.

Pressure is applied to the reaction system in order to maintain the hydrocarbon substantially in the liquid phase. The exact pressure applied, therefore, is dependent upon the temperature maintained in the reaction zone. Generally, the reactor will be pressurized to at least p.s.i.ga., a minimum pressure of 125 p.s.i.ga. usually being required to maintain the liquid phase at the temperatures used in the process. The maximum pressure employed is governed only by the mechanical limitations of the available equipment. However, because the use of extremely high pressures, i.e. above 1000 atmospheres, does not result in any advantages from the standpoint of yield, this figure represents the practical upper limit of pressure.

The reactor can be pressurized with any inert gas, inasmuch as the specific inert gas used is not critical. Instead of the nitrogen which was exemplified, carbon dioxide and helium also can be used.

In the carrying out of the nitration, a molar excess of the hydrocarbon is used, the excess hydrocarbon acting simply as a diluent which prevents localized concentrations of side reactants and, consequently, excessive side reactions. The extent of the excess of the hydrocarbon is not critical to the process of the present invention, the unreacted hydrocarbon being recovered and reused without deleterious effects upon the economics of the process.

Since tthe reaction proceeds so rapidly, good results are obtained in a matter of minutes and extensive reaction periods are not required. Thereby, long exposure of the resultant nitro compounds to elevated temperatures, which is accompanied by decomposition of these compounds, is eliminated. n-Propyl and isopropyl nitrate were exemplified as the nitrating agents because of their ready availability; however, any alkyl nitrate is suitable for use as the nitrating agent, the nitrate group of the molecule being the portion of the molecule which efiects nitration. By the term alkyl nitrate as used throughout the specification and the claims is meant the saturated aliphatic nitrates and their analogs, the alicyclic nitrates. Other suitable nitrating agents include ethyl nitrate, n-butyl, isobutyl, and sec-butyl nitrates, mixed amyl nitrates, hexyl nitrate, and cyclohexyl nitrate, among others.

The process of the present invention is general for alkanes, cycloalkanes, and aryl-substituted alkanes which can be maintained in the liquid phase under the reaction conditions. In addition to the hydrocarbons used in the experiments described in the foregoing examples, various other hydrocarbons can be nitrated by the process of the present invention. For example, additional nitro paraffins can be prepared readily from the corresponding parafiinic hydrocarbons, e.g. n-hexane, neohexane, and isooctane. Other cycloalkanes which can be nitrated by the present invention include, among others, cycloheptane and cyclopentane. The term cycloalkane as used throughout the specification and claims also includes, of course, alkylsubstituted cycloalkanes such as the exemplified methylcyclohexane and others, e.g., alkylated cycloheptanes and cyclopentanes. Additional aryl-substituted alkanes suitable for use in the process of the present invention are, for example, toluene, ethylbenzene, cumene, cymene, durene, and diisopropylbenzene. Furthermore, nonhydrocarbon substituents, such as nitro or halo groups, present on the hydrocarbons to be nitrated do not interfere in the carrying out of the process of the present invention, and compounds such as nitrocyclohexane, chlorodecane, bromopentane, etc. constitute suitable starting materials. Of course, when a nitro-substituted hydrocarbon is used as the starting material, a polynitro derivative is obtained which contains an additional nitro group.

Although the preceding examples illustrate the process as a batchwise process, equally feasible is the carrying out of the process in a continuous manner. For example, the

alkyl nitrate and the hydrocarbon may be introduced continuously into a reaction zone maintained at the desired operating conditions, while the product mixture is continuously withdrawn.

The invention has been described in detail in the foregoing. It will be apparent to those skilled in the art that many variations are possible without departure from the scope of the invention. For example, the hydrocarbon to be nitrated may be admixed with a diluent, e.g. benzene or acetonitrile, which is inert to the alkyl nitrate under the reaction conditions. I intend, therefore, to be limited only by the following claims.

I claim:

1. A process for the preparation of aliphatic nitro derivatives of alkane, cycloalkane, and simple mononuclear aryl-substituted alkane hydrocarbons which comprises mixing a lower alkyl nitrate with a molar excess of said hydrocarbons in a reaction zone maintained at a temperature in the range of about 140-300 C. and at a pressure of at least about p.s.i.ga., said hydrocarbon being maintained substantially in the liquid phase.

2. Process according to claim 1, wherein the lower alkyl nitrate is a propyl nitrate.

3. Process according to claim 1, wherein the reaction zone is pressurized with an inert gas selected from the group consisting of nitrogen, helium, and carbon dioxide.

4. Process for the preparation of nitrocyclohexane which comprises mixing a lower alkyl nitrate with a molar excess of cyclohexane in a reaction zone maintained at a temperature within the range of and 300 C. and a pressure of at least 125 p.s.i.ga., said cyclohexane being maintained substantially in the liquid phase.

5. Process for the preparation of nitro derivatives of methylcyclohexane which comprises mixing a lower alkyl nitrate with a molar excess of methylcyclohexane in a reaction zone maintained at a temperature within the range of 140 and 300 C. and a pressure of at least 125 p.s.i.ga., said methylcyclohexane being maintained substantially in the liquid phase.

6. Process for the preparation of a-nitro-p-xylene which comprises mixing a lower alkyl nitrate with a molar excess of p-xylene in a reaction zone maintained at a temperature within the range of 140 and 300 C. and a pressure of at least 125 p.s.i.ga., said p-xylcne being maintained substantially in the liquid phase.

7. Process for the preparation of nitro derivatives of n-nonane which comprises mixing a lower alkyl nitrate with a molar excess of n-nonane in a reaction zone maintained at a temperature within the range of 140 and 300 C. and a pressure of at least 125 p.s.i.ga., said nnonane being maintained substantially in the liquid phase.

8. A process as in claim 2 wherein the lower alkyl nitrate is n-propyl nitrate.

9. A process as in claim 2 wherein the lower alkyl nitrate is isopropyl nitrate.

No references cited. 

1. A PROCESS FOR THE PREPARATION OF ALIPHATIC BITRO DERIVATIVES OF ALKANE, CYCLOALKANE, AND SIMPLE MONONUCLEAR ARYL-SUBSTITUTED ALKANE HYDROCARBONS WHICH COMPRISES MIXING A LOWER ALKYL NITRATE WITH A MOLAR ESCESS OF SAID HYDROCARBONS IN A REACTION ZONE MAINTAINED AT A TEMPERATURE IN THE RANGE OF ABOUT 140*-300* C. AND AT A PRESSURE OF AT LEAST ABOUT 125 P.S.I.GA., SAID HYDROCARBON BEING MAINTAINED SUBSTANTIALLY IN THE LIQUID PHASE. 